1. Field of the Invention
The present invention relates to a detecting device, adapted for use in a displacement detecting apparatus for non-contact measurement of the amount of displacement of a moving object (such as a stage, a belt, paper or fluid) or the moving velocity thereof, either by irradiating the moving object with a laser beam and detecting the variation in frequency of scattered light subjected to Doppler's shift according to the moving velocity of the moving object, or by introducing a coherent light beam into a diffraction grating on a movable scale mounted on the moving object, causing interference between the lights of specified orders of diffraction diffracted by the diffraction grating, to form interference fringes and counting the number of interference fringes.
2. Related Background Art
In the field of detecting devices for non-contact and precise measurement of the moving velocity of the moving object, there are known active systems and passive systems.
As the displacement detector of the active system, there is already known a linear encoder utilizing the diffraction grating interference, as shown in FIG. 1.
Referring to FIG. 1, a coherent light beam from a laser 7 is converted into a substantially parallel beam by a collimating lens (not shown), and introduced into a beam splitter 8 by which the beam is split at a face 8a into a transmitted light beam and a reflected light beam. The two light beams from the beam splitter 8 are respectively reflected by mirrors 9a, 9b and introduced into a diffraction grating 6 on a moving object 101 in such a manner that .+-.m-th order diffracted lights from the diffraction grating 6 are emitted substantially perpendicularly thereto.
More specifically, the split light beams are so introduced as to satisfy a condition: EQU .theta..sub.m .congruent.sin.sup.-1 (m.lambda./P)
wherein P is pitch of the diffraction grating 6, .lambda. is wavelength of the coherent light beams, m is an integer, and .theta..sub.m is the incident angle of the coherent light beams into the diffraction grating 6. Thus, the .+-.m-th order diffracted lights, emerging substantially perpendicularly from the diffraction grating 6 are superposed and guided to detection means 10.
Thus, the detection means 10 detects the interfered lights, or the pulses indicating the number of interference fringes corresponding to the moving state of the diffraction grating 6.
The linear encoder shown in FIG. 1 detects the moving state, such as the moving velocity or moving direction, of the moving object 101, utilizing thus detected pulses.
Such displacement detector of an active system is capable of highly precise detection of a submicron change in position, but it is difficult to construct such a detector smaller than a cube with dimensions of several centimeters, because a semiconductor laser and mirrors are involved in the configuration.
FIG. 2 is a schematic view when the displacement detector is miniaturized by integration of the components shown in FIG. 1. In FIG. 2 there are shown an active layer 11 of a semiconductor laser, mirrors 12a, 12b, lenses 13a, 13b, 13c for converting the laser beam into plane wave, a diffraction grating 6, and detection means 14. The working principle is same as in the device shown in FIG. 1.
In the detector shown in FIG. 2, the active layer 11 of the semiconductor laser 7 is parallel to the plane of the drawing, and the light-receiving face of the detection means 14 is formed parallel to the plane of the drawing.
Such configuration is difficult for miniaturization, because of a large number of components.
Also in such configuration, the laser beam emitted from the semiconductor laser spreads more in a direction perpendicular to the plane of the drawing, but less in a direction parallel to the plane of the drawing.
FIG. 3 is a schematic view of the laser beam emitted from the active layer of the semiconductor laser. The dimension of the active layer 2, in a cross section perpendicular to the beam emitting direction, is usually less than a micron in the direction of thickness and several microns in the lateral direction. Therefore, because of diffraction, the laser beam spreads more in the direction perpendicular to the active layer (direction of thickness) and less in the direction parallel thereto, rather inversely to the cross sectional shape of the active layer.
For this reason, in the displacement detector shown in FIG. 2, the light beam spreads significantly in the direction perpendicular to the plane of the drawing and overflows the width of diffraction grating 6, perpendicular to the direction 6c of pitch, thereby resulting in a lowered diffraction efficiency.
In order to suppress such spreading of the laser beam, it has been conceived to provide the configuration shown in FIG. 2 with a refractive power in the direction perpendicular to the plane of the drawing, but the means for providing such refractive power is very difficult to produce in practice.